ONLINE HEATING BTU CALCULATOR

Accurate heat calculations are essential for proper HVAC performance in residential and commercial spaces. While BTU requirements can be calculated manually using formulas, our BTU heat calculator and unit converter provide a faster and more reliable way to size heating and cooling equipment.

BTU Heat Calculator

BTU (British Thermal Unit) Converter

What Is a BTU and Why Does It Matter?

A British Thermal Unit (BTU) is a standard measurement of thermal energy. One BTU represents the amount of heat required to raise the temperature of one pound (0.453 kg) of water by one degree Fahrenheit.

Although a single BTU is relatively small—roughly equivalent to the heat from a match—it becomes meaningful when applied to heating and cooling systems. HVAC equipment is rated by BTU output, with higher values indicating greater heating or cooling capacity.

BTUs are also used to compare energy content in fuels. Agencies such as the Energy Information Administration rely on BTU conversions to evaluate and compare different fuel sources, including gases and oils.

How to Calculate BTU Requirements

BTU calculations are based on room size, ceiling height, and the temperature difference between indoors and outdoors. Follow these steps to estimate heating requirements:

1. Measure the length and width of each room.
2. Calculate the square footage of each space and total the values.
3. Determine the average ceiling height and multiply it by total square footage.
4. Find the desired temperature increase by subtracting outdoor temperature from the target indoor temperature.
5. Multiply the results by the sealed-building factor (0.135).

BTU Formula:

Square footage × ceiling height × temperature increase × 0.135 = required BTUs per hour

Example:

A 2,000 sq. ft. home with 9 ft. ceilings and a 30°F temperature increase requires:

2000 × 9 × 30 × 0.135 = 72,900 BTUs

In this case, the HVAC system should be rated to deliver at least 72,900 BTUs to meet heating demands.

As this example shows, using an online BTU calculator is significantly quicker and less error-prone than performing the calculations manually.

Understanding Temperature Increase

The temperature increase—sometimes called the temperature differential—is a key factor in BTU calculations. Here’s how to determine it:

1. Identify the typical lowest outdoor temperature for your area (historical winter averages are best).
2. Decide on the indoor temperature you want to maintain using your thermostat.
3. Subtract the outdoor temperature from the desired indoor temperature.

Example:

If the outdoor temperature is 50°F and the indoor target is 80°F, the temperature increase is:

80 − 50 = 30°F

Your Complete HVAC Sizing Resource

With the formulas above, you can accurately estimate heating or cooling energy needs for a room or entire building. For faster results, our BTU calculator and energy conversion tools handle the math automatically.

You can also convert between BTUs, kilowatts, calories, joules, and therms to compare energy requirements across different HVAC systems.

Browse our full selection of HVAC, plumbing, and electrical equipment—including thermostats, relief valves, and heating components—to ensure your system is sized correctly and operates efficiently.